Like mark said use a press! unless it's still in the engine....then good luck! it come's off with some effort. Try using some PB blaster to help a little bit and lightly tap it with a hammer if you can. BE VERY CAREFUL not to damage the gear it's really easy to with a hammer...this is really not the best way but it will get it off.
Why are you removing the gear? are you replacing the cam or is the gear already damaged?
I removed mine with the engine in. Don't remember taking it off being a struggle, but I really remember putting the new one back on being a huge pain. Pushing it on far enough to clear the circlip groove was a battle.